The invention concerns a method for manufacturing high melting point glasses with volatile components, in particular glasses from the group of boron glasses and borosilicate glasses.
It is stated in the book "ABC Glas" (Dr.-Ing. H.-J. Illig; .COPYRGT. Deutscher Verlag fur Grundstoffindustrie GmbH, Leipzig; 1991; 2nd edition) on page 36 under the heading "Borosilikatglaser" ("Borosilicate glasses") that boron glasses and borosilicate glasses used for technical applications and for laboratory glassware with high temperature strength have, on the one hand, low coefficients of expansion, but, on the other hand, require high melting temperatures that reach the limits of current furnace design.
The heating of boron glasses and borosilicate glasses results in the volatilization of boron oxides and alkali metal oxides, and in the tendency towards segregation or phase separation (reference: "Glaschemie;" Prof. Dr. W. Vogel; .COPYRGT. Springer-Verlag; 1992; 3rd edition; pp. 305-307), and also to recrystallization, all of which are processes which lead to disturbances in the melting process.
Due to these characteristics, most known furnace types cannot be used to melt boron glasses or borosilicate glasses. The best currently available possibility is the basic design of the so-called unit melter, which is a single-chamber furnace with a rectangular furnace tank and a plain crown, from which no intermediate walls extend towards the melt in the heating area.
Furnaces of this type are described in DE-B-2 034 864, in U.S. Pat. No. 2,800,175, in U.S. Pat. No. 2,890,547 and in U.S. Pat. No. 3,353,941. However, it has been shown that without modification, such unit meters are not well-suited for the melting of borosilicate glasses.
The book by Trier, "Glasschmelzofen--Konstruktion und Betriebsverhalten" (Prof. Dr. W. Trier; .COPYRGT. Springer-Verlag; 1984), specifies and illustrates on page 11 a borosilicate furnace that is operated with crown temperatures of 1650.degree. C. and higher, and in which a barrier wall is installed between the melting and refining areas on the one hand and the homogenization area on the other hand. However, there are no indications concerning the length of the barrier wall in the direction of flow or concerning the spacing of the upper edge of the wall from the melt surface. In addition, the wall is disposed behind the refining area and is thus not a refining bank, and is also much too short for adequate refining. Details of unit melters (cross-fired furnaces) are also specified on pages 133 and 154.
A similar melting aggregate is also known from EP-B-0 410 338, in which a step with two rows of bottom electrodes is arranged in front of a barrier wall. However, the wall is not a refining bank because of its short length in flow direction. The step should have a maximum height of 100 mm, which is less than about 15% of the normal glass bath depth of about 800 mm. The step serves only to retain any metals that may be present, in order to avoid short-circuits between the electrodes. Resealable openings in the bottom in front of the step are used to drain such metals. Bubblers are not installed in front of the rows of electrodes. The electrodes create a drum-like circulating flow, whose surface component moves to the charging end of the furnace. In this way, with a reduced furnace temperature, an area of glass bath surface free of batch materials should be produced, and the glass flowing in the direction of the throat should be kept near the glass bath surface for a longer period.
In addition, it is to be remarked that the use of electricity to increase the glass bath temperature for better refining only succeeds effectively if the electricity is introduced in a region where there is no return-current, or only an insignificant return current. A strong circulating current would cause intensive mixing of the glass with other glass which is still being melted, and would thereby seriously jeopardize the temperature increase required for the refining.
From EP-B-0 317 551 and DE-C-39 03 016, it is known to install so-called refining banks in the refining area, above the level of the furnace bottom and to install horizontal electrodes in front of the refining bank in order to increase the refining temperature. However, the melting and refining areas are thereby separated by dividing walls extending downwards from the furnace crown, and these walls extend into the melt and are therefore exposed to high temperature loads, and are provided with cooling channels.
Bubblers can also be provided in the melting areas. However the bubblers have no influence on the processes in the refining area due to the dividing walls. As far as the flow of the melt is concerned, the melting and refining areas are connected with one another by means of narrow bottom throats, i.e. channels that do not extend across the complete width of the melting tank, and in which no return-current forms.
DE-C-39 03 016 thereby also refers to the possibility of processing strongly volatilizing glasses such as opal, lead and boron glasses, without however indicating how segregation and recrystallization could be counteracted.
DE-B-1 210 520 discloses a refining bank that lies higher than the furnace bottom in order to achieve a shallow bath depth, but said refining bank features an additional sill at its beginning, and in front of it a dam that protrudes out of the melt.
The conditions become increasingly unfavorable as the oxygen content in the oxidation gas increases up to a level of technically pure oxygen because the gas temperatures and the reactivity of the gases increase as a result of higher concentrations of contaminants although the gas volumes decrease. By this means, the refractory materials of the furnace and any other connected equipment are jeopardized.
EP 0 086 858 A1 describes a melting furnace for glass known as a "Deep Refiner.RTM.", in which a raised bottom area ascends at an angle to a shallow area between the melting area, in which bottom electrodes are installed, and a deeper refining area. However, there is no step or similar device. Vertical circulation currents in the form of several convective flow patterns are produced by bottom electrodes and, if required, additional bubblers installed in the melting zone. Further side wall electrodes are installed above the shallow area to produce the highest furnace temperature at this location. It is also possible to install more electrodes in the deeper refining area. The shallow area acts as an additional refining zone. The melting of high melting point glasses with volatile boron oxides and alkali metal oxides, and in particular of borosilicate glasses, is not described, nor is the supply of oxidation gases to the burners.
FR 2 737 487 A1 reveals a glass melting furnace for flat or float glass manufacture, heated mainly by burners with a high oxygen content. The furnace has an up-stream melting zone and a down-stream refining zone, whereby the bottoms of these zones lie on the same horizontal plane and these zones are of the same depth. Situated between the melting and refining zones, there is a raised bottom with a trapezoidal cross section, although possibly with concave sides, which stretches from side wall to side wall of the furnace. The trapezoidal cross section is described as an important characteristic. As a result of the combined effect of rows of electrodes installed on both sides of the raised bottom area and a row of bubblers installed up-stream of the first row of electrodes, two convective flow patterns of relatively cool glass are produced, which are clearly separated from one another, and between which a hot zone is created by means of bottom electrodes installed vertically above the raised bottom area. The object is to prevent a return flow of glass from the refining zone into the melting zone. The height of the raised bottom area should be a maximum of half, preferably approximately 1/4 to 1/3 of the glass bath depth. A shallower glass bath depth across the raised bottom area is expressly excluded, with reference to the possibility of heavy corrosion. The raised bottom area is therefore not a refining bank, over which a horizontal laminar flow is produced in a shallow bath, and in which area the residence time is increased. There is no step present in front of the raised bottom area, in which electrodes are installed. The production volume of the glass, which does not have a high melting point, should lie between 100 and 1000 tonnes per day. The energy requirement for the bottom electrodes is very high, for example 1500 kW. The separating effect of the combination of the raised bottom area and the bottom electrodes is so significant, that it is possible to change the colour of the glass melt in the shortest possible time.